1. Field of the Invention
The present invention relates to a method of detecting the number of revolutions of an internal combustion engine. More particularly, it is concerned with a method of detecting the number of revolutions of an internal combustion engine that is capable of detecting such number of revolutions accurately and easily by using a cylinder discrimination signal in the case where a top dead-center signal as a reference signal for detecting such number of revolutions cannot be detected.
2. Description of the Prior Art
Recently, various electronic techniques have been utilized for vehicles such as passenger cars. For example, a microcomputer is utilized to effect various controls, including control of the operation of an internal combustion engine and control of the ignition timing of an ignition device, whereby it is intended to improve the operating efficiency and performance of the internal combustion engine. In order that such an electronic control system can effectively control mechanical devices such as an internal combustion engine, it is necessary that various status quantities associated with the mechanical devices to be controlled be detected accurately and fed to the control system as basic informations for the control.
The number of revolutions of an internal combustion engine is one of the above-mentioned status quantities and usually it is detected on the basis of signals provided from a crank angle sensor. As a method of detecting the number of revolutions of an internal combustion engine on the basis of signals provided from a crank angle sensor, there is known a conventional method which is shown in FIG. 3, and as shown utilizes a top dead -center signal S.sub.1 produced when each piston reaches the position of the top dead center, a cylinder discrimination signal S.sub.2 produced when a predetermined cylinder reaches the time point of ignition and a separately produced clock signal S.sub.3 having a constant period, and in which the number of pulses of the clock signal S.sub.3 in one period (t.sub.1 or t.sub.2) of either the top dead-center signal S.sub.1 or the cylinder discrimination signal S.sub.2 is counted to detect the number of revolutions of the internal combustion engine. According to this conventional method, a signal processing is usually performed in such a manner that the number of pulses of the clock signal S.sub.3 is counted at every period t.sub.1 of the top dead-center signal S.sub.1, and when it becomes impossible to detect the top dead-center signal S.sub.1 by some cause or other, the number of pulses of the clock signal S.sub.3 is counted at every period t.sub.2 of the cylinder discrimination signal S.sub.2.
According to such conventional method of detecting the number of revolutions of an internal combustion engine, however, in the case of counting the number of pulses of the clock signal S.sub.3 on the basis of the period t.sub.2 of the cylinder discrimination signal S.sub.2, the number of counted pulses of the clock signal S.sub.3 becomes large because the period t.sub.2 is generally an integer multiple of t.sub.1 (four times as long as t.sub.1 in this embodiment), and particularly in a low revolution region of an internal combustion engine the value of the period t.sub.2 becomes so large that the number of counted pulses of the clock signal S.sub.3 is further increased, thus causing the counter to overflow.
On the other hand, in the method wherein the number of revolutions of an internal combustion engine is detected by using a program which has been made in advance so as to count the number of pulses of the clock signal S.sub.3 at the period of the top dead-center signal S.sub.1, in the case of counting the number of pulses of the clock signal S.sub.3 at the period of the cylinder discrimination signal S.sub.2, the number of counted pulses becomes four times as large as that in the pulse counting at the period of the top dead-center signal S.sub.1, thus requiring at every period t.sub.2 a processing 1 for reducing the number of counted pulses to one fourth as is shown in FIG. 4, which is not desirable in the algorithm for attaining more rapid operations.